The present invention relates to thermal targets, and in particular to thermal targets with a simple construction and high emissivity that simulates a blackbody.
Thermal targets are used as radiation sources, primarily for calibrating infrared detectors, which detect electromagnetic radiation in the infrared range, approximately 2 xcexcm to 1000 xcexcm. It is important to accurately calibrate an infrared detector in order for the detector to provide reliable information about the temperatures corresponding to the electromagnetic radiation detected by the detector. Because infrared detectors are subject to shift and drift, it is necessary to periodically recalibrate infrared detectors while in use.
A thermal target""s temperature and emissivity needs to be known as accurately as possible in order for a thermal target to be used as a calibration reference. Accurately knowing a thermal target""s temperature and emissivity establishes a baseline that correlates a known temperature to an amount of radiation. Such correlation allows an infrared detector to detect the amount of radiation and know what temperature that detected amount of radiation represents. Subsequently, when the infrared detector detects infrared radiation, the baseline radiation amount and corresponding temperature are used to calculate the temperature of the object emitting the infrared radiation. Radiated output, E, from an object is calculated using a modified Stefan-Boltzman equation:
E=xcex5"sgr"44xe2x80x83xe2x80x83(equation 1) 
where xcex5 is the calculated emissivity for the object, "sgr" represents the Stefan-Boltzman constant, approximately 5.670xc3x9710xe2x88x928 W/m2-K4, and T is the temperature of the object.
Using equation 1, any object could be used to calibrate an infrared detector if the temperature and the emissivity of the object are known. However, determining an object""s emissivity depends upon the temperature of the object, the surface characteristics of the object, the wavelength of the emitted radiation, the viewing angle of the object, and other factors and is therefore difficult to determine. Relying upon an object""s determined emissivity is not precise enough for accurately calibrating an infrared detector.
For accurate calibration of an infrared detector a thermal target that closely simulates a blackbody, i.e., a theoretical object that is a perfect absorber and emitter of radiation, i.e., xcex5=1, is used. Use of a blackbody simulator simplifies calculation of the total radiation emitted, E, from a thermal target by relying upon accurate determination of the thermal target""s temperature and the Stefan-Boltzman law;
E="sgr"T4xe2x80x83xe2x80x83(equation 2) 
where "sgr" represents the Stefan-Boltzman constant, approximately 5.670xc3x9710xe2x88x928 W/m2-K4 and T is the temperature of the thermal target.
The Stefan-Boltzman law accurately predicts the total radiation emitted for a blackbody at a known temperature. Because the use of thermistors and other temperature measuring devices allow accurate measurements of a thermal target""s temperature to within plus or minus 0.1 Kelvin, a blackbody simulator with a high emissivity, i.e., xcex5 greater than 0.99, provides the reliable correlation of detected radiation to temperature needed for calibrating an infrared detector. The better the emissivity of the blackbody simulator, i.e., the closer xcex5 is to 1, the more accurate an infrared detector calibrated using the blackbody simulator can be.
It would appear that a blackbody simulator could be any black object. Such a simulator has been used in the past, but correspondence to a true blackbody has been poor. Many blackbody simulators are formed by creating a cavity in a core material, the cavity forming an aperture on one side of the core. The aperture is used to simulate a flat blackbody, having the shape and size of the aperture. Particular cavity shapes are chosen to cause multiple reflections of radiation within the cavity, and eventual absorption by the cavity walls of the majority of the electromagnetic energy entering the aperture.
Thermal targets simulating black bodies employ multiple reflections in some sort of a cavity to enhance their effective emissivity. If the cavity surface coating of a blackbody simulator has an emissivity above 0.7, then most cavity shapes in commercial use in blackbody simulators result in a device in which the on-axis emissivity of the aperture exceeds 0.99. Primary differences between thermal targets are the shape of the internal cavity, and the finish on the surfaces inside the cavity. Other important parameters are the size ratio of aperture to volume, weight, complexity, uniformity of emissivity, and robustness.
Since thermal target designs attempt to maximize emissivity, one measure of the quality of a thermal target is how many nines the thermal target is capable of. An emissivity of 0.9, one nine, is usually unacceptable. Many thermal targets work in the range of 0.985 to 0.995, two nines. For accurately calibrating infrared detectors, an emissivity of at least 0.999, three nines, is required to ensure accurate measurements by the infrared device. Some laboratory grade instruments have emissivities in the four nine range.
A blackbody simulator with a large cavity and a very small aperture emits almost none of the radiation that enters the aperture because of the high ratio of volume to aperture size of the cavity, i.e., emissivity is in the three to four nine range. A high ratio of cavity volume to aperture size results in blackbody simulators with a large volume and a comparatively small aperture. Although such blackbody simulators have a high emissivity, they are often heavy, bulky, and not easily transported, making them ill suited for many applications requiring a blackbody simulator.
An additional disadvantage of high volume, small aperture blackbody simulators is that the aperture provides only a small field of blackbody radiation, making alignment of an infrared detector with the blackbody simulator critical. For example, in an airborn or spaceborn infrared detection system utilizing blackbody simulators to periodically calibrate the detector, jolts or bumps that occur during flight can misalign the detector from the blackbody radiation field, thereby disabling calibration of the detector during in-flight use. Decreasing the size of the volume, or increasing the size of the aperture, results in a blackbody simulator that is less heavy and less bulky, and/or has a larger blackbody radiation field, however, the emissivity is also lowered because the lower ratio of volume to aperture size allows more incoming electromagnetic radiation to escape the blackbody simulator.
Another drawback to current blackbody simulators is polarization of electromagnetic radiation that escapes the blackbody simulator. Such polarized output results in lower emissivities because some forms of polarized light have a lower emissivity than other forms of polarized light.
A further disadvantage of current blackbody simulators is that complex surfaces on the inside of a volume, although useful for preventing incoming light from escaping through the aperture, are difficult to manufacture and are often fragile. During mobile use of infrared detection systems, i.e., in-flight or in combat situations, the fragile surfaces may become damaged and reduce the emissivity of the blackbody. Such a reduction in emissivity causes the infrared detector in the system to become less accurate and less able to distinguish objects in cluttered infrared fields, i.e., where an object""s temperature is close to the temperature of its surroundings. Complex surfaces also result in random reflection of incoming light, which causes non-uniform emissivity for the blackbody simulator.
Considering the above drawbacks of current blackbody simulators, there exists a need for a thermal target that closely approximates a blackbody with a low ratio of volume to aperture and high emissivity. There exists a particular need for such a thermal target having uniform emissivity and non-polarized output. There is also a need for a thermal target that is simple and rugged.
An objective of the present invention is to provide a blackbody simulator that is easy to construct, rugged, and lightweight. Another objective of the present invention is to provide a blackbody simulator that contains a large aperture, and has a high emissivity. A further objective of the present invention is to provide a blackbody thermal target that does not emit polarized light.
These and other objectives are met by embodiments of the present invention, which provide a blackbody thermal target for use as a calibration radiation source for infrared detectors. The present invention provides a field of view where electromagnetic radiation traveling along the field of view enters a trap and undergoes multiple reflections off of substantially flat plates. The substantially flat plates absorb the electromagnetic radiation and reflect the un-absorbed remainder, eventually reversing the electromagnetic radiations direction of travel so that only a tiny residual of the entering electromagnetic radiation escapes the trap into the field of view.
Accordingly, one aspect of the invention relates to an electromagnetic radiation trap comprising substantially flat plates; and structure to hold the substantially flat plates in angled relation to one another. Of the plates, one plate is a primary plate; and electromagnetic radiation striking a surface of the primary plate is substantially absorbed by the primary plate with the remainder of the electromagnetic radiation being reflected onto a surface of another flat plate. The flat plates are arranged so that the diminishing electromagnetic radiation is reflected onto at least a surface of each flat plate and eventually strikes a flat plate substantially orthogonally, causing the remaining electromagnetic radiation to substantially retrace its path from plate to plate and emerge from the electromagnetic radiation trap traveling in the general direction from which the electromagnetic radiation entered the trap.
Another aspect of the present invention relates to an electromagnetic radiation trap comprising: a first substantially flat plate, a second substantially flat plate, and a third substantially flat plate, each having a top surface and a bottom surface. There is a finish on the bottom surface of the first plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. There is a finish on the top surface of the second plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. There is also a finish on the top surface of the third plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. The electromagnetic radiation trap also comprises structure to hold the substantially flat plates in angled relation to one another. As part of the angled relation of the plates to one another, the first plate and the second plate are tilted at approximately the same angle with respect to a common plane, and the top surface of the second plate views the bottom surface of the first plate. Another aspect of the angled relation of the plates to one another has the top and bottom surfaces of the third plate substantially orthogonal to the common plane; and the top surface of the third plate viewing the top surface of the second plate.
In certain embodiments, the electromagnetic radiation trap further comprises: a fourth substantially flat plate, and a fifth substantially flat plate, each having a top surface and a bottom surface. There is a finish on the top surface of the fourth plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation; and a finish on the top surface of the fifth plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. The first plate and the fourth plate are tilted at approximately the same angle with respect to the common plane, and the top surface of the fourth plate views the bottom surface of the first plate. Part of the angled relation of the plates to one another has the top and bottom surfaces of the fifth plate substantially orthogonal to the common plane, and includes the top surface of the fifth plate viewing the top surface of the fourth plate.
Another aspect of the present invention relates to an infrared detecting apparatus comprising: an infrared detector; and an electromagnetic radiation trap. The electromagnetic radiation trap comprises: substantially flat plates; and structure to hold the substantially flat plates in angled relation to one another. Of the plates, one plate is a primary plate; and electromagnetic radiation striking a surface of the primary plate is substantially absorbed by the primary plate with the remainder of the electromagnetic radiation being reflected onto a surface of another substantially flat plate. The substantially flat plates are arranged so that the diminishing electromagnetic radiation is reflected onto at least a surface of each flat plate and eventually strikes a flat plate substantially orthogonally, causing the remaining electromagnetic radiation to substantially retrace its path from plate to plate and emerge from the electromagnetic radiation trap traveling in the general direction from which the electromagnetic radiation entered the trap.
Yet another aspect of the present invention relates to an infrared detecting apparatus comprising: an infrared detector; and an electromagnetic radiation trap. The electromagnetic radiation trap comprises: a first substantially flat plate, a second substantially flat plate, and a third substantially flat plate, each having a top surface and a bottom surface. There is a finish on the bottom surface of the first plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. There is a finish on the top surface of the second plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. There is also a finish on the top surface of the third plate that absorbs electromagnetic radiation and reflects remaining electromagnetic radiation without scattering the remaining electromagnetic radiation. The electromagnetic radiation trap also comprises structure to hold the substantially flat plates in angled relation to one another. As part of the angled relation of the plates to one another the first plate and the second plate are tilted at approximately the same angle with respect to a common plane, and the top surface of the second plate views the bottom surface of the first plate. Another aspect of the angled relation of the plates to one another has the top and bottom surfaces of the third plate substantially orthogonal to the common plane; and the top surface of the third plate viewing the top surface of the second plate.
One of the advantages of the present invention is that the blackbody simulator is rugged and lightweight so that it is easily incorporated into sensors, such as spaceborn, airborn or mobile ground-based infrared detectors. Another advantage of the present invention is that the blackbody simulator does not emit polarized light. A further advantage of the present invention is the ability to obtain a high emissivity by increasing the number of reflections that incoming radiation goes through. Yet another advantage of the present invention is the small cavity volume compared to the size of the radiating apeture, which enables construction of a compact, lightweight blackbody simulator.
Additional advantages and other features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art on examination of the following, or may be learned by practice of the invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.